FIELD OF THE INVENTION
The invention lies in the automotive technology field. More specifically, the invention relates to a system for transmitting data between a satellite station and a central station in a motor vehicle.
In vehicle occupant protection systems in motor vehicles, for example air bag systems, seatbelt tensioning systems etc., one or more satellite stations, which are arranged for example in side regions of the motor vehicle and perform impact detection or early detection of accidents there, are provided in many cases in addition to a central station which performs impact detection. Such satellite stations transmit data to the central station when an impact or a situation where a risk of an accident is detected, with the result that the station can control suitable triggering measures. The satellite station can also be configured in such a way that it transmits messages (life signs) to the central station at regular intervals.
U.S. Pat. No. 5,712,784 (European application EP 0 693 401 A2) discloses a data transmission method in which the satellite stations are connected to the control unit of the central station via dedicated data lines. The necessary voltage supply to the satellite stations is not mentioned in any detail in that specification, but is usually provided via dedicated voltage supply lines. In the prior art method, the data are transmitted in the form of pulse-width-modulated signals which have a constant period and whose pulse duty factor represents information to be transmitted. The pules are applied directly to inputs of the central unit of the central station via the data lines. The expenditure on lines and wiring which is necessary for the transmission of data and the supply of voltage to the satellite stations is relatively high.
German published patent application DE 42 01 468 A1, upon which the instant invention is based, proposes a bus system with an integrated voltage supply for all the system subscribers, in particular for motor vehicles, in which all the bus system components are fed by a common, apparently unregulated voltage source. Each subscriber is provided with a current source which can be switched on and off. Switching them on and off enables current pulses to be generated in accordance with the data that are to be transmitted and which are fed onto the bus system.
German patent DE 42 27 577 C1 discloses a method for the bidirectional transmission of signals according to the master/slave principle. A line that connects the master and the slaves is used both for the transmission of signals and also to supply power to the slave modules. The master signal is voltage-encoded, whereas the slave signals are current-encoded. The master signal is transmitted continuously without interruption, even at times in which no data are being transmitted.
U.S. Pat. No. 4,621,170 discloses a data transmission device in which the supply voltage is made available by means of a battery. The data are superimposed in the form of current pulses which are generated by current sources which can be switched on and off.